As a method for converting an AC voltage into a DC voltage, the following two methods are generally known. A first method uses a diode bridge circuit and a smoothing capacitor. The diode bridge circuit rectifies full waves of the AC from the AC power supply. The smoothing capacitor smoothes the DC after the full-wave rectification is performed.
In the first method, in either case of the AC voltage is positive or negative, a current always flows through a series circuit of two diodes. At this time, in the two diodes, a power loss corresponding to a product of the current flowing through each diode and a forward voltage of the diode occurs.
In a second method, a power factor improving converter (PFC) is interposed between the diode bridge circuit and the smoothing capacitor of the first method. The power factor improving converter controls the current flowing through the AC power supply to be sinusoidal and controls the current to be equal to a voltage phase of the AC power supply.
Also, in the second method, since the current flows through the series circuit of two diodes when performing the full-wave rectification, the power loss is generated. In addition, since the current alternately flows through a field effect transistor (FET) and the diodes configuring the power factor improving converter, further loss occurs.
Furthermore, in the power factor improving converter, an output voltage has set to be higher than an input voltage from necessity to make a waveform of an input current be a sine wave. However, a required voltage at the load is not always higher than the input voltage. In this case, a step-down converter is connected to a subsequent stage of the power factor improving converter. Then, the voltage boosted by the power factor improving converter steps down to a desired voltage. The loss also occurs during the step down. An entirety of the power conversion apparatus is constituted by three stages of an AC-DC conversion, a DC-DC (boost) conversion, and a DC-DC (step down) conversion, and power conversion efficiency appears as a product of conversion efficiencies thereof. For example, if the efficiency for one stage is 0.95, the efficiency becomes 0.95×0.95×0.95=0.86 in the three stages. That is, although excellent conversion having the efficiency of 95%, the efficiency falls to 86% in continuous three stages. As described above, even if individual conversion efficiency is good, the conversion efficiency is significantly reduced by multiple stages.
Recently, a demand for saving electric power of an electronic apparatus is increased and it is also an essential condition that current harmonic noise is not emitted so as not to adversely affect on an external environment. Thus, it is required to achieve both improvement of the conversion efficiency of the power conversion apparatus that supplies power to the load and a suppression function of a current harmonic.
JP-A-2011-147277 is an example of the related art.